The Epsilon Eridani Factor

by Paul Gilster on June 17, 2010

When I was a kid, interstellar destinations were sharply defined. It seemed obvious that you didn’t even consider Alpha Centauri, because a double-star primary system surely wouldn’t allow stable planetary orbits. So you looked around for single stars. Moreover, these should be stars a lot like the Sun, so that when Frank Drake began SETI with Project Ozma, it made all the sense in the world to focus on Tau Ceti and Epsilon Eridani. Both were similar enough to our own star to suggest that they would have planets, and maybe one like ours.

For that matter, we had no idea in those distant days whether the Sun was a statistical fluke in having planets or simply a garden-variety star with a system that was all but inevitable. These days we keep finding interesting planets, but so far (other than perhaps in the Gliese 581 system) we haven’t found anything enough like the Earth to consider any nearby system an obvious target for an interstellar probe. All that may change, and swiftly, when we learn more about Alpha Centauri’s system, and if WISE finds a close brown dwarf. The latter two possibilities are going to be resolved within a few short years.

The Closest Probe Targets

The Project Icarus team — the interstellar probe study, not JAXA’s IKAROS sail — has been considering potential targets for their creation, just as the Project Daedalus crew did back in the ’70s (the glorious era of starship creation in the Mason’s Arms pub — there were giants in those days). Ian Crawford studies the matter in the Icarus blog, noting that a realistic maximum distance for an early probe is about 15 light years. This assumes a fusion starship design capable of 0.15c going after a target allowing a mission duration of no more than 100 years.

These are interesting numbers. As Crawford (University of London) notes, at present we know about 56 stars in 38 different stellar systems within this range. We have to be careful here, for not all stars within this volume of space have been discovered, and moreover, there are discrepancies, if slight, between the various catalogs of nearby stars. Crawford leans toward the RECONS (Research Consortium on Nearby Stars) catalog of one hundred nearest stars.

Now we find out just how tricky the target choice is. Let’s assume just for argument that the hunt for Alpha Centauri planets comes up dry. What’s the next possibility? It turns out that among the 56 closest stars, we have one spectral type A, which is Sirius, and one F (Procyon). Two G-class stars are available, Centauri A and Tau Ceti. Five K stars are possible, including Centauri B. But the overwhelming majority of nearby stars, 41 out of the 56, are M-class dwarfs. We round out the list with three white dwarfs and three probable brown dwarfs, although these numbers should increase with further WISE data.

The Case for ϵ Eridani

So far we know of planets around two of the 56 nearest stars, Epsilon Eridani (a K2 at 10.5 light years) and the M-dwarf GJ 674, which pushes our distance limit at 14.8 light years. So how about Epsilon Eridani? Crawford writes:

The planet orbiting epsilon Eri is a giant planet, with a mass about 1.5 times that of Jupiter. It has a highly eccentric orbit, which brings it as close to its star as 1.0 AU (i.e. the same distance as the Earth is from the Sun), to as distant as 5.8 AU (i.e. just beyond the orbit of Jupiter in our Solar System), with a period of 6.8 years. Although this would span the habitable zone (i.e. the range of distances from a star on which liquid water would be stable on a planetary surface given certain assumptions about atmospheric composition) for the Sun, this orbit lies wholly outside the likely habitable zone for a K2 star like epsilon Eri.

…being a gas giant, this planet itself it not a likely candidate for life, and its eccentric orbit wouldn’t help in this respect either (although it is possible that the planet may have astrobiologically interesting moons, perhaps similar to Jupiter’s moon Europa, which could in principle support sub-surface life).

Other planets in the Epsilon Eridani system? Maybe. An unconfirmed sub-Jupiter mass planet in a distant (40 AU) orbit may be there, and perhaps the system houses more Earth-like worlds. We’ll find out with further exoplanet investigation, and it should also be mentioned that Epsilon Eridani is circled by an interesting dust and debris disk. So we have to keep this young star on our list, even as we hold our thinking open about Alpha Centauri, and we have to realize how many other stars may soon be shown to have planets. Crawford again:

Clearly it would be of great interest if planets were discovered orbiting closer stars. Currently there have been no such planets discovered, but they are very likely to exist. Based on the detection rate to-date, and allowing for the known biases in the detection methods, it has been estimated that roughly 30% of main-sequence stars will have planets with masses less than 30 Earth masses. Thus, we might expect 16 or 17 of the nearest 56 stars to be accompanied by planets and, given the current lack of data on very low mass planets, it could easily be more. Although not targeted at any of the nearest stars, statistical results from the Kepler mission (which is looking for low-mass planets orbiting solar-type stars by the transit method…will greatly improve these estimates within the next few years.

Image: This artist’s diagram compares the Epsilon Eridani system to our own solar system. The two systems are structured similarly, and both host asteroids (brown), comets (blue) and planets (white dots). Epsilon Eridani is our closest known planetary system, located about 10 light-years away in the constellation Eridanus. Its central star is a younger, fainter version of our sun, and is about 800 million years old — about the same age of our solar system when life first took root on Earth. Observations from NASA’s Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Credit: NASA/JPL-Caltech.

Finding Out Where We’re Going

When I first started thinking about unmanned robotic flight to a nearby star, my unexamined assumption was that any such probe would deliver the first data we had about the presence of planets in that system. But that was twenty-five years ago, and now we’re in the exoplanet era. We’re building equipment of such sophistication that we can talk about detecting exoplanets from the ground, and future space-based missions will certainly do spectroscopic analysis looking for biosignatures on interesting nearby worlds. And all this will happen long before a true interstellar probe can be built, so we have plenty of time to work with. Unlike Daedalus (whose team chose Barnard’s Star because of a mistaken detection of planets there), the Icarus team knows enough to keep the parameters tight and wait for more information.

Crawford’s money is on Alpha Centauri in the end, not only because of its proximity but because it’s not a single target but three (assuming a sufficiently ingenious mission trajectory), each of a different stellar class. As for me, I’ll agree with Crawford most of the way, though I still hedge my bets by keeping an eye on WISE and the possibility of a brown dwarf within three light years. The beauty of all this, as I said above, is that we’ll have hard answers very soon.

This is sort of 1/2 on topic… I am just wondering if anyone in the community knows of a good 3D astronomy software that includes updates of newly found extra-solar planets and other objects. With all of these planetary discoveries that are happening so frequently it would be nice to get an easy to use updated 3D picture of where all these planets and systems are in relation to our location, especially in regards to nearby systems withing 100ly.

I enjoy an application for my iphone called Exoplanet by Hanno Rein. It sends me messages when every they update the data base to include new planets. Just recently it notified me of 6 new planets Carot-12b, 11b, 13b, 14b, 8b, and 10b. It is not 3D but gives graphics of light curve or radial velocity. It also charts position in star field. I don’t see a listing of distance but I suspect that could be obtained from any star database. It also allows you to do interesting graphs of semi-major axis, eccentricity, orbital period, mass, host star metalicity of any subgroup of panets.

When I was a teen, the L-5 Society was in its heyday, and we didn’t care if other stars had planets or not. As long as there were asteroid and comet belts, O’niell style space colonies could be built and that was all that was thought to be necessary.

BTW, the Epsilon Eridani system does look a lot like our own solar system. There might even be a planet in the habitable zone there. Of course that planet will be very young and maybe just out of its hadean period.

Actually the assertion that the giant planet orbiting Epsilon Eridani is in such a highly eccentric orbit is inconsistent with the presence of the innermost debris belt at 3 AU discovered by Spitzer. In fact, dynamical stability requires the planet to be in a low-eccentricity orbit with eccentricity below about 0.10, or the population of parent bodies for the observed dust would have been cleared out by now. That would make the planet a pretty good analogue for Jupiter.

@Bill B.
I usually use Celestia because it’s a really slick free 3D space exploration program. You can download packs that populate the galaxy with more stars, and some with planets. It can show orbits and the planet at it’s currently predicted location or it’s future or past one.

Hi Paul
So many stars to choose from! How does one decide? Of course if the radial velocity studies of Alpha Cen B get a hit, then the choice becomes pretty easy. I still wonder about Gatewood’s claim of planets around Lalande 21185 which he made in 1996, but never followed up with a discovery paper. At 8.3 ly it just beats epsilon Eridani, though it is a red-dwarf star.

Regarding Icarus and hydrogen fusion propulsion, as we study low energy particle collision energy physics, such as in nuclear fusion at the NIF, the FRIB which is being built, and other facilities here in the U.S, and abroad, we never know what the payoffs will be in high energy density materials.

Now protons are composed of two Up quarks and one Down Quark, however, the rest mass of these combined quarks is only about 0.5 percent to 1 percent of the rest mass of the proton, the other portion being the relativistic kinetic energy of the bound quarks as well as the inertial effects of the binding gluon fields. A proposed quark bomb would somehow release the binding and relativistic energy of the quarks, which in concept is a kind of anathema to modern QCD Standard Model Theorists. Such a device if safely miniturized might safely use hydrogen to produce a hydrogen fission rocket on steroids.

I can imagine even much more extreme rest mass specific energy density scenarios, but I would drift way off topic.

The point is that nuclear energy and its association with Project Icarus is a great precursor approach to doing interstellar probes and possibly manned star flight.

I am intrigued by Project Icarus because of the relatively small size of the craft and the compact nature of fusion propulsion systems.

I still wonder about Gatewood’s claim of planets around Lalande 21185 which he made in 1996, but never followed up with a discovery paper. At 8.3 ly it just beats epsilon Eridani, though it is a red-dwarf star.

I hadn’t heard about Gatewood’s claim — what more can you tell us? As for red dwarfs, this keeps the focus on the whole question of the stellar classes we’re going to keep on the list. If astrobiology is a big issue, then red dwarfs may barely make the cut, but brown dwarfs seem a poor choice. If habitable zones for life like ours are the issue, then we’re stuck until we find a likely planet. But what times we live in. I seem to get surprised every other day…

Talking of brown dwarfs and habitable planets, have you come across this article? It’d be interesting to come up with some scenarios for evolution on such a planet whose star decreases in luminosity as it ages (as opposed to more conventional stars that brighten as they age) – perhaps life might begin in the cloud layers of an initially Venus-like planet, moving to the surface as the atmosphere cools and the oceans rain out of the atmosphere, and finally moving to a more Europa-like state with the oceans frozen under an ice layer.

The article andy mentions is Andreeshchev and Scalo, “Habitability of Brown Dwarf Planets,” and no, I hadn’t seen this before. I look forward to reading it — thanks for the tip! The astrobiological scenario you mention is fascinating.

Speaking of Lalande 21185, this admittedly old reference from 1979 (the book “Planetary Encounters” by Robert M. Powers) says on page 329: “Lalande 21185 C is at 8.1 light-years, with a companion thought to be about the size of Jupiter. (However, on the previous page he also describes the two planets that Barnard’s star was then believed to have.)

Speaking of Lalande 21185, this admittedly old reference from 1979 (the book “Planetary Encounters” by Robert M. Powers) says on page 329: “Lalande 21185 C is at 8.1 light-years, with a companion thought to be about the size of Jupiter.” (However, on the previous page he also describes the two planets that Barnard’s star was then believed to have.)

All,
How about Epsilon Indi as a back up candidate to the Alpha Centauri system, which right now has to be considered the clear leader given proximity and also that it is a very interesting looking Star System? There are indications that Epsilon Indi has planets and that recent dating further indicates that it is a much older star then previously thought. A second back up candidate after Epsilon Indi could be 61-Cygni. Beyond these, and the leading candidate Alpha Centuari, things get pretty meager until we get out to about 18-2o LY where one finds Gliese, Sigma Draconis the 36 Ophiuchi and 70 Ophiuchi systems, and a batch of G stars with Eta Cass (Achird) being the most promising. Virtually everything else has no detectable planets such as Tau Ceti (but keep looking just in case since it would be ideal), is to young to support any sort of Intelligent Life including our own if we tried to colonize it, or is likey to be overtly hostile to the sustainment of life such as Procyon, Delta Pavonis or KEID since they are in their Red Giant Phase.

The sad truth is that unless M-class stars turn out to be viable there are only a handful of interesting candidates out to ~20 LY from the Sol/Terra system which is why we need a detailed survey of all Star Systems out to 20 LY’s from Sol/Terra to narrow the problem through a process of knowledgable elimination. In the end it may turn out that it is either Alpha Centauri, Epsilon Indi or bust in terms of finding anything interesting to directly explore that we can reach with technology that may be available to us over the next ~100 years. As for traveling to a close proximity Brown Dwarf the question becomes why since we can explore this through detailed observation with a TPF type mission.

Further to andy’s comment on Eps Eri’s (lower) eccentricity, I read either on this site or Greg Laughlin’s systemic, that probably a lot of measured eccentricity is false and in reality consists of (an)other planet(s) hidden in the data.
As solar type candidates for habitable terrestrial planets within 20 ly I would mention, beside the already mentioned Eps Eri and Tau Ceti:

The first four seem to have rather low metallicity (Tau Ceti also, btw). Besides, Sigma Draconis is probably a variable star. Furthermore, Delta Pavonis, though not a red giant yet (to Kenneth Harmon), is in the process of moving off (or at least into a later stage of) the main sequence, getting quite hot and bright for this spectral type star, about 1.24 times solar luminosity. Eta Cassiopeiae A is part of a binary, but the minimum separation is about 36 AU, a luxury compared with the Alpha Centauri system.

Chi1 Orionis A is a barium dwarf, so maybe not very suitable and Beta Comae Berenices is on the very bright side (about 1.4 – 1.5 times solar lum.).

My absolute favorites here are Beta Canum Venaticorum and 61 Virginis, particularly the latter, my no. 1 within 30 ly, because it seems to have everything going for it, such as right metallicity, spectral type, luminosity, long-term stability, etc. I was therefore quite disappointed when, last year, I learned about it having a super-earth (5 Me) and two Neptunes class planets (about 17 and 21 Me) in close orbit, resp. 0.5, 0.22 and 0.48 AU. It is still possible that it possesses a terrestrial planet (or moon) in the habitable zone (centered around 0.7 AU, about Venus distance in our solar system).

A bit beyond that, at 33 ly, is my other great favorite and similar to 61 Vir: Alpha Mensae, also G5 and all the right parameters. No planet detections yet, as far as I know.

This looks very impressive and promising, talking about technological advancement and surprise! Does this also mean that terrestrial planet detection and direct imaging plus spectro-analysis will become feasible with ground-based systems, making space telescopes and interferometers largely redundant?

Another factor making it difficult to determine the orbital parameters of Epsilon Eridani b is the high level of activity on the star. That makes radial velocity measurements difficult, I’m not sure how it would affect astrometry (though only a small portion of the orbit was covered by astrometric measurements).

Incidentally according to its SIMBAD page, Alpha Mensae has a companion red dwarf star at roughly 30 AU projected separation, discovered in 2007.

Easier said than done! If we want information return within 100 years the absolute extreme for range is 50 ly – anything past that needs to be travelling FTL. Of course achievable rocketry means the range is a lot closer to Sol within the century timespan. Just as an example of the difficulty involved let’s assume a fusion rocket with a 0.1 c exhaust velocity – a fusion rocket builder’s dream goal. To get information from a target with a probe doing ~0.5c the maximum 100 year range is 33.3 ly. A fusion rocket that carries fuel to accelerate, but uses a magnetic sail to deccelerate, would need a mass-ratio of 243 to reach 0.5c. That’s not impossible using multiple stages. Robert Goddard’s 1919 black-powder moon-rocket design had a huge mass-ratio too. He achieved it by using a tower of small stages which fell away in clusters. Similar ideas were used by the BIS for their man-carrying version of 1939 – a dubious prospect safety-wise.

Realistic fusion rockets will get exhaust velocities in the 0.03-0.05c range, though one can fantasise about the higher range. If we can push a probe to 1/3c, with a huge mass-ratio, then the data-return speed, at most, is 0.25c. No doubt there are other ways to propel a probe to the stars, but just what is an open question. As much as I find discussions of laser/particle beam propulsion interesting they do have issues like the need for immense power and very precise focussing. They’re not a project for the early days of interstellar travel, though I like to imagine otherwise.

Ronald: it’s the projection of the separation in the sky plane: we can measure the angular separation between the two and convert that to a distance in the sky plane, but the relative distance along the line of sight is unknown. If, say, the companion is 100 AU or so closer to us or further away than Alpha Mensae, we can’t detect that. That means 30 AU is a minimum separation at the time of observation.

The orbital properties are (as far as I am aware) completely unknown, we haven’t observed the system for long enough to make an accurate estimate. So we can’t rule out the case that it is near the apastron of a highly eccentric orbit that brings it within a few stellar radii of the primary at periastron, although this is probably quite unlikely.

A flyby mission to a nearby star would get there in the least time, for a given amount of fuel, but it sends the probe past the target very fast. On the other hand, a rendezvous mission, where you turn the vehicle around halfway and take back all the velocity you gained in the first half, takes longer but allows you to go into orbit around the target star. But a flyby has the further advantage that you can use the gravity of the target star to redirect you toward the next target. You can take a tour of nearby space. Wouldn’t that be cool? More about this on my blog.

ok if there was a earth like planet in the Epsilon Eridani System we should call the planet REACH

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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